Direct fuel-injection diesel engines generally have a piston with a top face formed so as to be flat, but a direct fuel-injection diesel engine in which a piston top face projects in a pentroof shape is known from Patent Publication 1 below.
When a cavity is recessed in the top face of a pentroof-shaped piston, the height of the opening of the cavity changes in the circumferential direction. Therefore, if the height of a bottom wall portion of the cavity is made constant in the circumferential direction, the depth of a peripheral wall portion of the cavity changes in the circumferential direction, the conditions in which fuel injected from a fuel injector and air are mixed becomes nonuniform in the circumferential direction, and there are the problems that engine output decreases and harmful exhaust materials increase.
In order to solve these problems, in the arrangement described in Patent Publication 1, the height of the bottom wall portion of the cavity is changed so as to follow the change in the height of the opening of the cavity, thus making the depth of the peripheral wall portion uniform in the circumferential direction, and thereby making the conditions in which fuel and air are mixed within the cavity uniform in the circumferential direction. Patent Publication 1: Japanese Patent Application Laid-open No. 62-255520